1. Field of the Invention
This invention relates to a controller for managing a series of compressors to provide a head within a dead band with the most efficient use of the compressors to reduce power consumption and lower maintenance costs.
2. Description of the Related Art
When a series of compressors supply air pressure for pneumatic tools and equipment in a factory, shop or other facility, the compressors frequently have staggered, independent, set point controls, which turn on or off additional compressors, or load or unload the compressors, as the head decreases or increases. Since the compressors are not well coordinated, and the motors have long time delays before they shut off when the compressor is unloaded, two or more unloaded compressors can be running at one time, wasting energy and placing unnecessary wear on the motor and compressor.
Staggered set points are usually employed to load and unload compressors at descending and ascending pressure as its ability to sequence multiple units. It is better for the motors and compressors to start and stop a limited number of times per hour. Unless long time delays are employed on motor shut down, it will result in too many motor cycles per hour. Long time delays are therefore used resulting in wasted energy by running two or more motors unloaded at the same time.
During the day demands for air pressure will vary resulting in high amplitude sine waves of pressure in the pressure tank. With staggered set point systems the sine waves can trigger two or more compressors to unload. Since each unloading and loading will change the compressor output by about 80%, the fluctuations in pressure in the tank will change more than is required forcing more cycling than is necessary. It is generally better to follow a large change, for example 80%, with a small change, for example 20%, to smooth the fluctuations. If, however, with this prior art, the delays were simply shortened to shut down the motors to implement the 80%-20% sequence, there would still be too many motor cycles, since it is only part of the solution.
For initial starting of the compressors, individual controls on staggered set point compressors may turn on more compressors than needed during the start up for the day or in deep amplitude sine wave cycling. Most compressor systems do not keep track of how long each compressor is on and what the motor""s current use is for tracking system management and cost savings statistics.
High rates of change in head may trigger several compressors to unload at the same time when there are independent set points for the compressors. Therefore several compressors may be in an unloaded state for long periods of time when the motors have long delay times. Therefore the system does not react efficiently to large changes in head.
The invention uses a controller with one set point for controlling all of the compressors in the system. The controller limits the number of compressors turned on at one time to the minimum number needed to supply the load on the system and limits the number of compressors running unloaded to one compressor at a time. This is accomplished by not only adding and shedding loads by pressure offsets but also making use of information gathered continually on the system pressure, the amount of departure from the set point and the rate of departure, its reversal rate of return to the set point, whereby a dynamic change in add or shed decisions are on-going to prevent add or shed decisions that are either late or premature. The number of compressors turned on and their load states, by this means, are coordinated to produce the most efficient use of the compressors, which saves energy and reduces wear on the compressors.
The controller uses a shorter time delay between unloading the compressor and shedding the motor for the compressor than the prior art, avoiding long periods of time with a compressor running in an inefficient unloaded state when the head has increased.
The controller avoids having two or more unloaded compressors at a time by using the special aforementioned logic. Thus the cycling of the motors and loads is minimized and the amplitudes of the sine waves of pressure differences is reduced. Limiting large numbers of start-ups and shut-downs with this method limits wear on the motors and compressors and reduces maintenance costs while saving energy.
The controller can detect rapid changes in head to shed or add compressors or loads more quickly and efficiently with dynamically changeable time delays for different rates of change in head, to keep the system near the set point and in some cases it will stop and hold the countdown to add or shed when the rate of pressure departure from the set point slows up, to determine in another brief period of time whether the loading will increase or decrease, hence more intelligent add/shed decisions are made. In most cases these last second decision changes will prevent a xe2x80x9cshort cyclexe2x80x9d.
The controller also coordinates the sequencing of the compressors such that they have approximately even numbers of run hours and start-ups. The number of motor startups per hour is counted to limit the number of motor start-ups in accordance with the compressor manufacturer""s recommendations.
Where some compressors employ multiple stages of loading the controller can then load the compressor with varying degrees with the same method and obtain fine tuning for the head.
The initial start up is limited usually to one or two compressors, lowering the electrical current used and preventing unnecessary cycling prior to the plant commencement of loading the system.
The controller is easily installed on existing systems and programmed for the particular needs of the facility.
The control panel allows for ease of programming and use of either the original equipment controls or automatic controls via this invention.
It is an object of the invention to avoid having multiple compressors running unloaded at the same time.
It is an object of the invention to coordinate the operation of all compressors in a system.
It is an object of the invention to save energy in a system having a plurality of compressors.
It is an object of the invention to limit wear on the compressors.
It is an object of the invention to minimize the number of start-ups and shut-downs of the compressors.
It is an object of the invention to provide run time data for each compressor.
It is an object of the invention to alternate the lead/lag sequence of the compressors for even wear.
It is an object of the invention to let the operator select either the OEM controls, manual controls or the controller of the invention for operating the compressors.
It is an object of the invention to run the minimum number of compressors to maintain tank pressure at the set point.
It is an object of the invention to provide a controller that is easy to install on a compressor system.
It is an object of the invention to easily program the controller.
It is an object of the invention to increase the life of the motors and the compressors.
It is an object of the invention to limit the number of on/off cycles per motor during a given time period.
It is an object of the invention to keep track of motor current consumed on each motor and for the system so as to calculate the savings that the controller provides.
It is an object of the invention to limit the initial start up to one compressor or as few as will satisfy the time desired to reach the set point.
It is an object of the controller to respond more quickly in loading the compressors for higher rates of drops in pressure.
It is an object of the controller to shed compressors and loads on the compressors faster for higher rates of increases in pressure.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.